The invention concerns a device and a method to homogenize in a working volume of a magnetic resonance apparatus the magnetic field created by its main magnet. The working volume is surrounded by a multitude of ferromagnetic platelets whose number, magnetization strengths and fixed positions are chosen such that in combination they essentially cancel field inhomogeneities of the main magnet across the working volume.
Such a device and such a method are known for example from EP 0 272 411 B1.
In contrast to magnet shimming with so-called shim coils which are fed with correction currents, recently in particular in magnets of imaging nuclear magnetic resonance (MRI) the so-called "passive shimming" has more and more become dominating where the actively controllable shim coils are replaced by passive, ferromagnetic or also by permanent magnetic small platelets being placed around the working volume at predetermined locations. In common superconductive tomography magnets with room temperature bore, rod-shaped support structures are inserted into axial guiding mechanisms at the wall of the room temperature bore. In each support structure, at predetermined positions, a pre-calculated number of ferromagnetic "shim platelets" is stacked and fixed.
Whereas for example in superconducting high resolution NMR magnets shim coil sets are still in use in order to homogenize the field at the sample location, in the field of nuclear spin tomography (MRI) in most cases the above mentioned method of passive shimming is used which requires more calculation effort but is less expensive and is on the long run more convenient since it has to be performed only once and needs later on no current or power supplies.
However, even when rather small individual shim platelets are used, the field correction is in a certain sense "digitized" since at predetermined positions only platelets with fixed magnetic moment can be added or removed. Moreover, this procedure requires access to the working volume itself or at least reconstructions in the immediate surroundings of the working volume. Therefore passive shimming can fully profit from its advantages only if the field homogenizing can really be performed once and for all, if need be in a few iteration steps. In cases where the field distribution across the working volume changes with time, e.g. already by introduction of a measuring object, by the time-dependent influence of ferromagnetic objects in the neighborhood or also by a change of the main coil field profile during operation, in particular of permanent magnets or resistive magnets with pole pieces, readjustment at a later time can be desirable.
In EP 0 823 641 A1 it is suggested to use passive shim platelets at predetermined locations as well as shim coils in order to limit on the one hand the number of required shim positions and on the other hand the required shim currents. This requires both, installation of a conventional shim system with coils and support structures for passive shims.
U.S. Pat. No. 4,682,111 discloses a pole piece magnet, each pole piece comprising a concave portion surrounded by a toroidal portion. To further improve field uniformity, a plurality of hydraulically adjustable, ferromagnetic projections extend from the surface of each pole piece. The number of these projections is fixed and they are moved by complex spring mechanisms integrated into the ferromagnetic pole pieces comprising pressurized channels through the pole piece material of the electromagnet.
There is a need for a homogenizing device and a homogenizing method of the kind mentioned initially, making use of passive shim platelets but which allow in a second step a fine adjustment of the field homogenizing by means of these platelets without requiring a re-distribution or a reconstruction of the support structure.